This invention relates to a method and system for non-contact measurement of a gap between a sensor and a conductive or non-conductive surface using a capacitive measurement device with a plurality conductive plates that permits measurement of material depth and dielectric changes in solids and fluids.
Non-contact gap measurement sensors having two parallel superimposed conductive plates, which are electrically insulated from one another, are disclosed in, for example, U.S. Pat. Nos. 4,675,670; 5,990,807; 6,075,464 and 6,552,667. A high frequency signal is placed on the first plate of the sensor. By measuring the capacitance between the sensor and a proximate surface, the sensor generates a signal that is indicative of the gap between the sensor and the surface.
A difficulty with existing non-contact capacitive sensors is that the capacitive signal generated by the sensor that is representative of the gap may be overshadowed by noise. The noise may arise from capacitance variations of extension cables between the sensor and its associated electronics, signal pickup electronics and transformer, and stray capacitances from the signal pickups. The noise due to these capacitance variations may be much greater than the capacitance of the signal indicative of the gap.
Another difficulty in using a capacitive sensor is that capacitance is formed between the sensor and any surfaces which come near the sensor, including surfaces behind the sensor. Some sensitivity to surfaces behind the sensor remains even when an active shield plate is placed behind the sensor plate.
There is a need for a capacitive measurement method and a non-contact capacitive measurement sensor that is less sensitive to variations in capacitance, in an insulator between the two plates of the sensor and variations in the impedance of the cables connecting the sensor to the circuit. Excessive sensitivity to these variations may increase the difficulty in manufacturing the sensor and increase the sensor sensitivity to temperature and other environmental factors.